This invention applies to the combustor section of gas turbine engines used in powerplants to generate electricity. More specifically, this invention relates to the structure that transfers hot combustion gases from a can-annular combustor to the inlet of a turbine.
In a typical can-annular gas turbine engine, a plurality of combustors are arranged in an annular array about the engine. The combustors receive pressurized air from the engine""s compressor, add fuel to create a fuel/air mixture, and combust that mixture to produce hot gases. The hot gases exiting the combustors are utilized to turn a turbine, which is coupled to a shaft that drives a generator for generating electricity.
The hot gases are transferred from each combustor to the turbine by a transition duct. Due to the position of the combustors relative to the turbine inlet, the transition duct must change cross-sectional shape from a generally cylindrical shape at the combustor exit to a generally rectangular shape at the turbine inlet. In addition the transition duct undergoes a change in radial position, since the combustors are rigidly mounted radially outboard of the turbine.
The combination of complex geometry changes, rigid mounting means, as well as high operating temperatures seen by the transition duct create a harsh operating environment that can lead to premature deterioration, requiring repair and replacement of the transition ducts. To withstand the hot temperatures from the combustor gases, transition ducts are typically cooled, usually by air, either with internal cooling channels or impingement cooling. Severe cracking has occurred with internally air-cooled transition ducts having certain geometries that are rigidly mounted to the turbine inlet and operate in a high temperature environment. This cracking may be attributable to a variety of factors. Specifically, high steady stresses in the region around the aft end of the transition duct exist where sharp geometry changes occur and a rigid mount is located. Such a rigid mount located at the transition duct aft end does not allow for adequate movement due to thermal growth of the transition duct. In addition stress concentrations have been found that can be attributed to sharp corners where cooling holes intersect the internal cooling channels in the transition duct. Further complicating the high stress conditions are extreme temperature differences between portions of the transition duct.